The present invention relates to an airbag which is structured to restrict gas flow from the airbag through a vent hole. Further, the present invention relates to an airbag apparatus comprising the airbag.
It is well known to form a vent hole in an airbag to release gas from the inside of an inflated airbag, whereby the vehicle occupant is softly received by the airbag.
Japanese Unexamined Patent Application Publication No. 2005-199987 (Patent Document 1) discloses an airbag in which a vent hole is covered by a lid member. In this publication, a proximal end portion, near an inflator (the center of the airbag) in relation to the vent hole, of the lid member is unreleasably connected to the airbag. In addition, a distal end portion, on the other side of the vent hole from the inflator, of the lid member is caught at the periphery of the airbag between a front panel (occupant side surface) and a rear panel (non-occupant side surface) of the airbag and is releasably sewn together by a tear seam. The tear seam is formed by a sewing yarn which is designed to break when the inner pressure of the airbag exceeds a predetermined value.
According to the airbag of the publication, at an initial stage of inflation of the airbag with gas from the inflator, the connection of the front panel and the rear panel of the airbag and the lid member by the tear seam is not released so that the lid member overlaps with the vent hole. Therefore, the gas flow through the vent hole is restricted, thereby achieving rapid inflation of the airbag.
When the inner pressure of the airbag then reaches the predetermined value, the tear seam breaks so that the front panel and the rear panel of the airbag are separated and the lid member is released from the front panel and the rear panel. Because of the inner gas pressure of the airbag, the lid member is pushed out of the airbag through the vent hole so that the vent hole is opened. Therefore, when the vehicle occupant collides with the inflated airbag, the gas flows from the inside of the airbag to the outside of the airbag through the vent hole. As a result, the vehicle occupant is softly received by the airbag.
According to the airbag disclosed in the Patent Document 1, the vent hole can be opened only by break of the tear seam.
A threshold value of strength for the tear seam to break strongly depends on temperature and is thus affected by heat of gas spouted from the inflator. According to the location, the output, and the like of the inflator, the timing of the break of the tear seam tends to vary.
It is an object of the present invention to provide an airbag which does not use a tear seam, while the airbag is still capable of maintaining a vent hole in the closed or slightly opened state at an early stage of inflation of the airbag and is also capable of opening or largely opening the vent hole when gas generation of the inflator is decreased or ended after the early stage and also provide an airbag apparatus comprising the airbag.
Further objects and advantages of the invention will be apparent from the following description of the invention.